Polarity in oxide ultrathin films.

Relying on first-principles calculations within density functional theory and on an analytical model for the electronic structure, we present an overview of specific electronic and structural features of polar ultrathin films. MgO(111) unsupported films of finite thickness are chosen as a generic system, in order to extract general concepts associated with polarity at the nanoscale, relate them to the well-known semi-infinite case, and unravel specific scenarios of polarity compensation which are not present for the latter. Size dependent behavior of the compensating charge and formation energy, changes in crystallographic structure, and the possibility of substantial lattice distortions throughout entire films are analyzed and discussed.